I'm about halfway through the game right now (maybe 8 hours of play), and I must say, this is among the handful of best first-person shooters I've ever played! The level design is excellent, the 4-channel sound is spectacular, there are scripted events happening constantly to liven things up, and it has this wacky mix of elements from other games and movies (the female computer from System Shock, sounds from the movie Forbidden Planet, etc.).
I just finished No One Lives Forever, which is one of the year's most highly reviewed games, and I am definitely finding Gunman to be more addictive.